I don't have any pictures of these dreaded saplings. Each day I must inspect my soil to remove any new acacia saplings
It seems that you are doing the best thing, already, if there are other things in the ground that you do not want to kill, nearby.
If the area has nothing important that you don't want to kill, then you can try boiling water to kill them off. Here is an article that will help:
https://www.gardeningknowhow.com/plant-problems/weeds/boiling-water-and-plants.htm
You can also try solarizing the soil. This article will help:
the stem with the boom gets longer and longer if I cut that stem will that kill the possiblity of another bloom?
Unfortunately, your photo did not come through. I am unable to assess the damage.
It sounds like an infection, which in container is usually caused by overwatering. Cutting without treating can result in worsening of the condition.
I would recommend a fungicide, and to make sure that the soil in the container dries down to about 2 or 3 inches between waterings.
Here are some articles that will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/ornamental/flowers/popcorn-cassia/popcorn-cassia-information.htm
I am interested in growing KOA outside the state of Hawaii. I have selected a few locations in Mexico and in Nicaragua. Do you have anyone in your organization you can refer me to? I will obtain the seeds from Maui and the Ancient Northern forests of the big island. I will want to grow them to harvest for wood to make building finishes and furniture. Any help would be greatly appreciated.
They are short-lived outside their native region of Hawaii. We don't have anyone to refer you to, but here is an article we have on the Koa tree:
I am looking to purchase Acacia tree seeds, cuttings or small saplings. Where can I purchase any of these?
We don't recommend vendors or products, but your local extension agent can help with locating plants or seeds, as well as a local nursery. Here is info about acacia.
I planted this shoe string acacia 2 years ago. I live on the central coast of California. Watered regularly the first year and followed instructions from the nursery. However, it’s not growing. Please help!!
Make sure it is still alive by scraping off some of the bark. If it is green underneath, it is OK. If it's brown, it has succumbed.
You can remove any dead branches. Have you fertilized? You can continue to water monthly. These articles should help:
https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-scratch-test.htm#search
https://sactree.org/trees/1003/
https://climatereadytrees.ucdavis.edu/northern-california-central-valley-trees
https://www.public.asu.edu/~camartin/plants/Plant%20html%20files/acaciastenophylla.html
https://apps.cals.arizona.edu/arboretum/taxon.aspx?id=9
https://cals.arizona.edu/forageandgrain/sites/cals.arizona.edu.forageandgrain/files/az1150-2015.pdf
If the soil is not well draining, it can get root rot.